There should be £26 annual savings for customers in the north west, but just £5 a year for households in the south east.

Dermot Nolan, Ofgem chief executive, also admitted that costs to consumers of using other energy networks, such as those for gas and electricity transmission and gas distribution, would together rise by £8 next year.

However, this should still leave a £4 net reduction in costs that energy companies should pass on to consumers.

"Overall, the consistent and clear message is that network costs are not rising, despite the significant investment, network costs are - if anything - falling," he said.

News of costs falling will pile further pressure on the Big Six energy suppliers to cut their tariffs, following sustained falls in wholesale gas and power costs.

But there is no guarantee cuts will actually result from Ofgem's announcement, with suppliers already privately complaining about increases in other elements of the bill.

A spokesman for Ofgem said: "In a competitive market we would expect suppliers to pass on this saving to consumers."

Mr Nolan said the £17bn it had provisionally approved would help to prepare the networks to cope with households installing new green technologies such as heat pumps as well as solar panels and other "small-scale renewable generation"

The £17bn was £2.1bn lower than the five companies had sought to spend in plans last year.

The companies had shaved £700m off the plans themselves after being told to go back to the drawing board, while the regulator had cut a further £1.4bn, Ofgem said.

A sixth company, Western Power Distribution, had its spending plans approved last year.

How much should your bill fall by?

Reductions vary for customers of different companies, serving different regions.